Death of neurons by apoptosis represents a common underlying cause of or contributor to brain diseases such as stroke and neurodegenerative disorders. Apoptosis can be induced by multiple stimuli that engage specific pathways leading to activation of Caspase-family cell death proteases. Pathways initiated by TNF-family death-receptors and by mitochondria have been elucidated in detail in recent years, but other pathways remain uncharacterized. Caspase activation and apoptosis often result from stress to the Endoplasmic Reticulum (ER), though the responsible mechanisms are unclear. During the previous funding period, we identified anti-apoptotic proteins that associate with the ER, including BI-1. The BI-1 protein contains 6 predicted transmembrane domains and resides predominantly in the ER. Over-expression of BI-1 by gene transfection renders cells more resistant to apoptosis triggered by ER-stress inducing agents, such as Thapsigargin and Tunicamycin, while having comparatively little effect on apoptosis induced via ER-independent pathways. Conversely, targeted ablation of the bi-1 gene in mice results in a selective increase in cellular sensitivity to apoptosis induced by ER-stress agents in vitro and in vivo. Interestingly, expression of the bi-1 gene is induced by hypoxia, suggesting a role for this anti-apoptotic protein in adaptation to ischemia, and ablation of the bi-1 gene in mice resulted in hypersensitivity to stroke injury. In this competitive renewal application, we propose to test the hypothesis that BI-1 represents a pivotal regulator of ER-stress pathways connected to apoptotic responses. Using transgenic mice over-expressing BI-1 and knock-out mice lacking BI-1, as well as cells derived from these animals, we propose to: (1) Dissect the point in apoptotic pathways where BI-1 blocks cell death induced by ER-stress; (2) Determine the effects of BI-1 on ER-dependent functions such as Ca2+ sequestration; (3) Define the mechanisms that control bi-1 gene expression in response to hypoxia; and (4) Determine the role of BI-1 in resistance to stroke during ischemia-preconditioning and focal ischemia stroke models in genetically engineered mice. Altogether, these studies will improve understanding of how damage to the ER is linked to apoptosis, and will potentially reveal strategies for neuronal preservation in stroke and neurodegenerative diseases.